Field of the Invention
Our invention relates to a multilayer belt that can be used for creping a cellulosic web in a paper making process. Our invention also relates to methods of making paper products using a multilayer belt for creping in a papermaking process. Our invention still further relates to paper products having exceptional properties.
Related Art
Processes for making paper products, such as tissues and towels, are well known. In such processes, an aqueous nascent web is initially formed from a paper making furnish. The nascent web is dewatered using, for example, a belt-structure made from polymeric material, usually in the form of a press fabric. In some papermaking processes, after dewatering, a shape or three dimensional texture is imparted to the web, with the web thereby being referred to as a structured sheet. One manner of imparting a shape to the web involves the use of a creping operation while the web is still in a semi-solid, moldable state. A creping operation uses a creping structure such as a belt or a structuring fabric, and the creping operation occurs under pressure in a creping nip, with the web being forced into openings in the creping structure in the nip. Subsequent to the creping operation, a vacuum may also be used to further draw the web into the openings in the creping structure. After the shaping operation(s) is complete, the web is dried to substantially remove any remaining water using well-known equipment, for example, a Yankee dryer.
There are different configurations of structuring fabrics and belts known in the art. Specific examples of belts and structuring fabrics that can be used for creping in a paper making process can be seen in U.S. Pat. No. 8,152,957 and U.S. Patent Application Publication No. 2010/0186913, which are incorporated herein by reference in their entirety.
Structuring fabrics or belts have many properties that make them conducive for use in a creping operation. In particular, woven structuring fabrics made from polymeric materials, such as polyethylene terephthalate (PET), are strong, dimensionally stable, and have a three dimensional texture due to the weave pattern and the spaces between the yarns that make up the woven structure. Fabrics, therefore, can provide both a strong and flexible creping structure that can withstand the stresses and strains of operation on the papermaking machine during a papermaking process. Structuring fabrics, however, are not ideally suited for all creping operations. The openings in the structuring fabric, into which the web is drawn during shaping, are formed as spaces between the woven yarns. More specifically, the openings are formed in a three dimensional manner as there are “knuckles,” or crossovers, of the woven yarns in a specific desired pattern in both the machine direction (MD) and the cross machine direction (CD). As such, there is an inherently limited variety of openings that can be constructed for a structuring fabric. Further, the very nature of a fabric being a woven structure made up of yarns effectively limits the maximum size and possible shapes of the openings that can be formed. And, still further, designing and manufacturing any fabric with specifically configured openings is an expensive and time-consuming process. Thus, while woven structuring fabrics are structurally well suited for creping in papermaking processes in terms of strength, durability, and flexibility, there are limitations on the types of shaping to the papermaking web that can be achieved when using woven structuring fabrics. As a result, it is hard to simultaneously achieve higher caliper and higher softness of a paper product made using creping operations.
As an alternative to a woven structuring fabric, an extruded polymeric belt structure can be used as the web-shaping surface in a creping operation. Unlike structuring fabrics, openings of different sizes and different shapes can be formed in polymeric structures, for example, by laser drilling or mechanical punching. The removal of material from the polymeric belt structure in forming the openings, however, has the effect of reducing the strength, durability, and resistance to MD stretch of the belt. Thus, there is a practical limit on the size and/or density of the openings that may be formed in a polymeric belt while still having the belt be viable for a papermaking process. Moreover, almost any monolithic polymeric material (i.e., a one layer extruded polymeric material) that could potentially be used to form a belt structure will be less strong and stretch resistant than a typical structuring fabric, due to the nature of a monolithic material in comparison with a woven structure.
Attempts have been made to use polymeric belt structures with an extruded polymeric layer in papermaking operations. For example, U.S. Pat. No. 4,446,187 discloses a belt structure that includes a polyurethane foil or film that is attached to at least a woven fabric for reinforcing the belt. This belt structure, however, is configured for use in dewatering operations in the forming, press, and/or drying sections of a papermaking machine. As such, this belt structure does not have openings of a sufficient size to perform web structuring, such as that in a creping operation.
An additional constraint on any creping belt or fabric to be used in a papermaking process is a requirement for the creping belt or fabric to substantially prevent cellulose fibers used to make the paper product from passing through the creping belt or fabric during the papermaking process. Fibers that pass completely through the creping belt or fabric will have a detrimental effect on the papermaking process. For example, if a substantial amount of fibers from the web is pulled completely through the creping belt or fabric when a vacuum from a vacuum box is used to draw the web into the openings of the creping structure, the fibers will eventually accumulate on the outside rim of the vacuum box. As a result, caliper of the paper product will substantially decrease due to air leaking from the seal between the vacuum box and the creping structure. Also, the accumulated fibers, which result in an unwanted variation in the paper product properties, will also have to be cleaned off of the outside rim of the vacuum box. The cleaning operation results in expensive down time for the papermaking machine and lost production. In general, it is preferable that less than one percent of the fibers should pass completely through the creping belt or fabric during a papermaking process.